It sounds like you are asking about a couch that is sagging? This can occur when the springs on the bottom of the couch have come loose or have been stretched beyond their designed capacity (Usually the kids playing trampoline on it!)

Remove the cushions from the couch and then turn the couch over to expose the underside. When you turn your couch over, it may have a fabric covering that will have to be removed before any repairs can be made.

Inspect the springs and the fasteners. If the springs have come loose, it should be an easy fix. Straighten out any bent or miss-shapened springs.

Re-fasten any loose springs if possible or add some additional wood slats for more support.

To increase the tension in the springs, use an 7/16-1/2 inch hemp rope, secure it to the top of the springs and weave it thru the springs from top to bottom, fastening it at the bottom and top of each spring, keeping the rope in one continuous piece.

It may be necessary to replace some of the springs and attachments if they are broken or usable. Check with your local furniture repair shop to see if you can purchase replacement springs and fasteners for your model couch.

There are other ways to fix a sagging couch, but the above, are my favs. Please let us know if you have any further questions.